1. Field of the Invention
The present invention relates to a data memory device and, more particularly, to a ferroelectric memory device which stores data in a ferroelectric capacitor.
2. Description of the Related Art
A general ferroelectric memory has a cell structure in which data is stored in the ferroelectric capacitor of each cell. Data read is performed by applying a voltage to the ferroelectric material to cause polarization reversal, and sensing a charge amount extracted outside. This charge amount extracted outside is temperature-dependent; the higher the temperature, the smaller the output charge amount. This is so because the spontaneous polarization amount in the ferroelectric material is temperature-dependent.
The relationship between the voltage and polarization of the ferroelectric capacitor has a hysteresis, and the shape of the hysteresis loop of the ferroelectric capacitor at room temperature differs from that at a high temperature. Therefore, the spontaneous polarization amount of the ferroelectric capacitor reduces when the temperature rises.
Accordingly, a signal output from a memory cell of the ferroelectric memory during data read decreases as the temperature rises. If the absolute value of the signal becomes close to the performance limit of a sense amplifier, a read error is highly likely to occur.
In the conventional ferroelectric memory, cell data is read by an external instruction regardless of the device temperature (e.g., S. Sheffield Eaton et al., “A Ferroelectric Nonvolatile Memory”, IEEE International Solid State Circuit Conference 1988 Technical Digest p. 130). The conventional ferroelectric memory also has the problem that data is read even when the device temperature is high and a read error may occur (e.g., Jpn. Pat. Appln. KOKAI Publication No. 2001-332082 or 2001-210080).
In addition, since the ferroelectric memory is a destructive read device, wrong data is rewritten if a read error occurs. Accordingly, the memory has the problem that stored data itself may be destroyed, i.e., data which is not destroyed if it is not read may be destroyed.